1. Field of Invention
This invention relates to devices utilized to feed liquid explosives into boreholes as part of a mining operation. More particularly, the present invention pertains to a wand which can be positioned near a lower collar opening over the borehole and which operates to seal the opening against outflow of the liquid explosive during loading.
2. Prior Art
The success of underground mineral mining operations depends on efficient fracturing procedures and ore extraction. One form of fracturing operation is referred to as sublevel caving and involves fracturing overhead rock formations in a controlled manner. A complete and continuous fracturing process is important to maximize ore recovery and avoid the high risk of unexpected collapse of overhead rock structure into the sublevel cave area.
The required systematic fracturing of rock structure is accomplished in part by drilling a set of substantially upright, overhead boreholes which are loaded with explosive material. These boreholes may be fan-shaped or parallel in orientation, depending on the desired pattern of collapse of superior rock structure.
The challenge to loading a near-vertical hole from a bottom collar upward is to pump liquid explosive up the length of the hole without allowing the material to drain free of the collar. In the past, mechanical plugs have been used around the insert wand which is positioned within the collar opening. Such plugs are effective; however, they are time-consuming to use and difficult to adapt to all hole configurations. In essence, each mechanical plug must be sized to the differing shape of collar openings. This added time substantially increases the cost of overall mining operations in view of the many boreholes which muswt be filled for a single blasting.
Inflatable wands have been utilized which provide an annular balloon chamber around the flow conduit or wand which inflates with air to seal the surrounding collar opening. Inflation is developed by compressed gas fed to the chamber. This type of inflatable wand requires a rigid conduit to prevent its collapse in response to the inflating air pressure of the balloon chamber. Such rigidity, however, limits the ability of the wand to bend or deform slightly for entry within a collar opening which is angled. It also increases diameter of width requirements of the cavern to at least match the length of the wand to be inserted.
In addition, the prior art gas-inflatable wand requires the use of accessory equipment. This adds capital costs and further time to the loading operation which reduces process efficiency. Eventual removal of the plug also limits selection of explosive materials to those with a high viscosity, a resistance to shear and capacity for integral wall contact to prevent collapse of the loaded explosive with loss of support.
The absence of a more versatile method for sealing the collar opening of the borehole has remained an unsolved problem for many years and resulted in substantial inconvenience and increased costs. A more desirable system would enable the use of a greater variety of explosives, less expensive hardware and the capacity to load quickly and combine the explosive material within the borehole during pumping, blocking venting of material through the collar.
Other unrelated fields of industry have addressed the problem of expandable bladders for use within flow lines but have not provided sufficient relevant teachings or structure to lead to the development of the desired wand or other form of acceptable product. For example, the plumbing industry utilizes an expandable bladder which can be inserted within a pipe. The bladder comprises a simple balloon element connected in line with a pressurized water source and a restrictive nozzle at its end. The device is placed within a pipeline that is clogged and water flow is activated. The restrictive nozzle increases the water pressure within the balloon element, causing it to enlarge and seal the void space at the conduit within the pipe opening. Pressurized water then flows into the flow line and dislodges foreign material therein.
A similar type device is applied within the cement industry for backfilling cracks and subsurface cavities below an existing slab of concrete. The latter device comprises a delivery hose with surrounding air bladder which is inflated to operate as a plug. It is placed within a hole drilled through the slab and the cement is pumped and migrates under pressure to fill void spaces. The air bladder is inflated within the drilled hole to seal it against backflow of the pressurized grout or cement.
Neither of these devices has been successfully applied within the subject mining industry. From a practical viewpoint, this prior art structure is simply not suited to the rigorous environment of a mining operation. In contrast with the smooth interior surface of a pipe or drilled concrete, borehole walls are characterized by irregular sharp points and edges that can quickly cut and tear elastic materials needed for the ballooning effect. To insert the bladder elements of either the plumbing or grouting within the mining borehole interior and to apply pressure would result in expansion, movement and a shredding reaction of the bladder wall against the sharp surfaces of the borehole wall. In other words, the very expansion that is intended to block against liquid backflow, causes bladder rupture.
Such unpredictable response is simply not acceptable when working with large quantities of highly explosive material. As a result, the mining industry continues to rely on use of the mechanical plug or air pressurized device which operates independently of the pressure within the flow line. In each case, the liquid explosive is not involved in expansion of the plug or bladder. It merely passes through the wand and into the borehole interior and column. Backflow and leakage of the liquid is reduced by insertion of plug material or by use of a separate, air-inflated bladder around this flowline conduit.